Typical linear magneto-resistive (xMR) sensors for the detection of rotational speed are based on the measurement of an in-plane magnetic field component (e.g., x-component) along an axis defined by the running direction of a magnetic or ferromagnetic material passing the sensor. Under ideal orientations of the magnetic field axis to the sensor structure and with no unintended magnetic field components present, the linear xMR sensors can be designed to be highly precise and to have low hysteresis effect. In reality, however, several effects may impact the signal quality of the linear xMR sensors.
One effect that may impact the signal quality is phase shifted in-plane magnetic fields along an axis perpendicular to the desired measuring (i.e., x-component) magnetic field axis. For example in a pole wheel application, depending on the adjustment of the sensor structure, magnetic field components in the y-direction (i.e., y-components parallel to the axis of the pole wheel) may be present that are phase shifted by approximately 90° to the x-component magnetic field axis. Therefore, a rotating pole wheel passing the sensor structure results in a magnetic field vector rotating circularly or elliptically.
Another effect that may impact the signal quality is misalignment of the sensor structure due to a twisted chip in the sensor package (which is related to the package tolerance) or misalignment of the sensor structure to the desired magnetic field axis (which is related to the adjustment tolerance). The misalignment leads to a non-desired angle between the sensor structure and the axis of the magnetic field to be measured.
These effects can lead to hysteresis and jitter in the output signal of a linear xMR sensor designed for detecting only a single magnetic field component. Hysteresis and jitter adversely affect the performance of the xMR sensor, thereby complicating the development of further fields of application for the xMR sensor. Typical anisotropic magneto-resistive (AMR) sensors reduce hysteresis and jitter by using a constant auxiliary magnetic field having a component perpendicular to the magnetic field to be measured. As long as the disturbing y-component magnetic fields are lower than the x-component magnetic field to be measured, the y-component magnetic fields have no negative influence on the hysteresis behavior. The auxiliary magnetic field is typically provided by an auxiliary magnet mounted externally beneath the sensor structure. The auxiliary magnet increases the cost of the AMR sensor. In addition, there are magnetic field limits that may not be exceeded to guarantee proper sensor functionality.
For these and other reasons, there is a need for the present invention.